In the course of electro-dipcoating, also called electrophoretic painting, the lacquer constituents in the lacquer bath are deposited on the work-pieces under the influence of an electric field. It has proved expedient not to keep the electric field constant during the passage of the work-pieces through the lacquer bath, but rather to vary it along the path of motion, in particular to cause it to increase with the distance travelled. One of the reasons for this measure is the fact that the lacquer layer that is being built up constitutes an electrical resistance which renders further build-up of the lacquer layer difficult.
Consequently the problem arises as to how the voltage for each work-piece can be changed in the course of passing through the lacquer bath. With electro-dipcoating apparatuses of the type mentioned in the introduction—such as are described in DE 199 42 556 C2, for example—the supply of current to the work-piece is effected via a contact rail that is divided into individual sections in the direction of motion of the work-pieces to be lacquered; the sections are galvanically isolated and are connected to a pole of a voltage source assigned to each section. The electrodes arranged along the path of motion of the work-pieces in the lacquer bath are connected to the other pole of the voltage sources. A disadvantageous aspect of this arrangement is that it is relatively difficult to track, by methods of control engineering, the path of each individual work-piece along the various sections of the contact rail and to bring about the transition of the work-piece from one section to another without a ‘jump’ in potential. Since the charge that has flowed between the electrode and the work-piece is used as a measure of the layer thickness applied, it is also necessary to switch the measurement of the current flowing to the work-piece concomitantly, section by section, with the motion of the work-piece. Finally, the ‘cuts’ in the contact rails, which bring about the galvanic isolation, also constitute surface irregularities in the contact rail, which result in wear of the contact devices that are moved concomitantly with the work-pieces.
In the event of a change in the type of work-piece—for example, a change in the length of the work-piece—or in the event of a change in the immersion curve, the positions of the cuts between the contact-rail sections change, so that a change of plant is required for an optical coating.